As an application of X-ray computed tomography (CT), a technique is available in which differences in X-ray absorption characteristics among materials are utilized to discriminate, for example, types, amounts, and densities of materials contained in a subject. This is called material decomposition. Increasing the monochromaticity of X-rays used for the material decomposition increases the difference in interaction between a certain material and other materials. For this reason, highly monochromatic X-rays, that is, X-rays with a narrow energy range are preferably used to perform the accurate material decomposition.
However, increasing the monochromaticity of X-rays used for the material decomposition reduces the number of photons of the X-rays, so that the material decomposition becomes susceptible to noise. The dose of X-rays irradiating the subject needs to be reduced, so that the number of photons of the X-rays is difficult to be increased to avoid the influence of noise. For this reason, the conventional material decomposition with the X-ray CT is difficult to accurately discriminate, for example, types, amounts, and densities of materials contained in the subject.